Now new research published in Nature Geoscience shows that such frozen Arctic soil holds nearly twice as much of the organic material that gives rise to planet-warming greenhouse gases as previously estimated.

"When the air temperature rises two to three degrees, the Arctic tundra would switch from a carbon sink to a carbon source," says soil scientist Chien-Lu Ping of the University of Alaska Fairbanks. "The greater the carbon stores, the greater the impact it causes," including even faster warming in the already changing Arctic.

Previous estimates of the amount of carbon stored in Arctic soil had relied on samples taken outside of the region or those from only the topmost layer—accessible by shovel. As a result, these surveys failed to account for organic material deeper down.

So Ping and his colleagues used portable jackhammers to chisel their way three feet (a meter) or more into a variety of landscapes in the Alaskan Arctic, 117 sites in all. Excavating these soil pits, the scientists found that permafrost areas were quickly storing plants and other organic material deep beneath the surface, as much as 10 feet (three meters) in some cases.

The key is the thawing and freezing of the surface layer as well as some of the unique soil formations of the Arctic region, according to Ping. "Frost boils"—deep soil that roils to the surface like bubbles in a boiling pot—and ground cracks allow topsoil to slip below the surface and come into contact with the deep permafrost. This keeps the soil from decomposing—and thereby traps more than 98 petagrams (98 million billion grams) of carbon, or one sixth the total in the atmosphere, according to biogeochemist Christian Beer of the Max Planck Institute for Biogeochemistry in Jena, Germany.

Warming could release most of that carbon. "Warming will increase the decomposition rate of the soil organic matter and more carbon dioxide will be generated," Ping notes. And "warming will thaw the permafrost and release the sequestered carbon," which will also decompose faster in the warmer temperatures.

It remains unclear how much of an impact such thawing permafrost will ultimately have and the sites sampled were solely in North America—leaving the vast tundra of northern Europe and Siberia to be an educated guess. But it is clear that adding even a fraction of the carbon dioxide or methane stored in Arctic soil to the atmosphere would have a "significant impact on Earth's climate," Beer says.